One of J.P. Morgan Chase & Co.'s top executives quit suddenly to take a top job in private equity, a departure that raises new questions about who will eventually succeed Chief Executive James Dimon.

Michael Cavanagh's decision to leave his job as co-head of J.P. Morgan's Wall Street operations marks the latest in a long line of departures from Mr. Dimon's inner circle. But the move still shocked many in the company, including Mr. Dimon, according to people close to him. The 48-year-old Mr. Cavanagh and Mr. Dimon,...